Systems and methods described herein generally relate to the field of ultrasound imaging. More specifically, embodiments described below relate to methods and systems for color flow imaging.
Ultrasound is used to image various internal structures, including but not limited to the heart, the liver, a fetus, and blood vessels. For diagnosis of cardiovascular diseases, color Doppler (or color flow) imaging is usually used to visualize blood flow in the heart or blood vessels. Abnormal conditions often increase blood flow velocity in comparison to that under normal conditions. The increased velocity may result in aliasing within a corresponding color Doppler image. Color Doppler uses a pulse ultrasound technology for its spatial sampling capability, which limits the maximum frequency which can be detected without experiencing aliasing. The pulse repetition frequency (PRF), which is also the sampling frequency, sets the maximum frequency limitation. This limitation, in turn, limits the maximum blood flow velocity which can be measured without exhibiting aliasing. In cardiac cases especially, the limitation is severe because the imaging depth is usually deep and thus PRF cannot be set high enough to measure abnormally high blood velocities, for example, regurgitation jets across heart valves. Therefore, in abnormal cardiac conditions, color Doppler often exhibits aliasing, thereby reducing the reliability of any diagnosis based on the blood flow image. Thus, there exists a need to overcome this aliasing problem.